Diffusion transfer photographic processes in which a silver salt such as a silver halide is used are well known. In such photographic processes, an exposed photosensitive element containing a photographic emulsion of a silver halide is laminated with an image receiving element containing silver deposition nuclei and the inner part between these two elements is then processed with coating an alkaline processing solution containing a silver halide solvent in the presence of a developing agent, thereby causing a positive silver image to be formed directly on the image receiving element. In other words, the un-exposed portion of the silver halide emulsion in the photosensitive element is dissolved in the silver halide solvent contained in the alkaline processing solution to form a silver ion complex which is then transferred to the image receiving element. The thus transferred complex compound is deposited as a silver image in the image receiving element by the action of the silver deposition nuclei to form a positive image directly.
The image receiving element for use in such processes is generally prepared by superposing an image receiving layer on a paper support such as baryta paper, polyethylene laminate paper, lacquer paper, synthetic paper or the like, or on a film support such as acetyl cellulose, polyethylene terephthalate, polystyrene or the like. In this instance, the image receiving layer comprises an alkali permeable polymer binder which is selected from the group consisting of gelatin, carboxymethyl cellulose, hydroxyethyl cellulose, regenerated cellulose, polyvinyl alcohol, sodium alginate, starch, gum arabic, colloidal silica and the like, and a compound dispersed in the polymer binder which forms silver deposition nuclei and which is selected from another group consisting of metal sulfides such as palladium sulfide, nickel sulfide, silver sulfide and the like, or noble metal colloids such as colloids of gold, silver, palladium and the like.
For the purpose of improving such image receiving elements, a large number of techniques have been developed. In some of these techniques, regenerated cellulose is used as a binder for the image receiving element. For example, U.S. Pat. No. 3,179,517 discloses a process for the formation of an image receiving element in which a regenerated cellulose layer is prepared by hydrolyzing an acetyl cellulose film with an alkali and then the regenerated cellulose layer is soaked in a gold salt solution and a reducing agent solution to form silver deposition nuclei of gold colloid in the layer. JP-B-44-32754 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses an image receiving element which is prepared by providing silver deposition nuclei in a polymer material hardly permeable to alkali by means of vacuum deposition, dissolving the resulting polymer material in a solvent, coating and drying the dissolved mixture on a support and then hydrolyzing the surface layer of the thus formed polymer layer to give the layer alkali permeability.
JP-B-46-43944 discloses a process for the preparation of an image receiving element in which acetyl cellulose containing silver deposition nuclei in a dispersed form is coated on a support and then hydrolyzed to transform it into regenerated cellulose. Also, U.S. Pat. No. 4,163,816 discloses an image receiving element which is prepared by subjecting a solution of acetyl cellulose to acid hydrolysis in order to modify the cellulose material into a low acetylation degree acetyl cellulose and then coating the thus modified acetyl cellulose on a support.
In general, unlike typical photographic processes, the temperature at the time of exposure is the developing temperature in the case of diffusion transfer processes which, therefore, require a control means so that the developing time can be lengthened or shortened to correspond to low or high temperatures. To provide such control means, a neutralization layer to neutralize the alkali in the developing solution and a neutralization timing layer to control the alkali-neutralizing rate corresponding to the temperature are applied to the diffusion transfer process. In such a case, cellulose esters are frequently used as the neutralization timing layer and are also sometimes used as a binder for the neutralization layer.
As described above, acetyl cellulose or regenerated cellulose is well known as a binder for image receiving elements for use in silver salt diffusion transfer processes and is a very useful material.
The image receiving element for use in silver salt diffusion transfer processes must have a shading function in order to provide a light-free space as a "dark room" at the time of the development step. In the case of image receiving elements for use in peel-apart type silver salt diffusion transfer processes, such a shading function is generally incorporated into the support or its backing layer. In this instance, incorporation of the shading function into the backing layer may be most convenient, because obtainment of a support material having a shading function is relatively difficult, whereas many kinds of support materials having no shading function are on the market and can be obtained easily.
Backing layers having a shading function have been disclosed, for example, in U.S. Pat. No. 3,752,692 and JP-A-61-48845 (the term "JP-A" as used herein means an "unexamined published Japanese patent application). With regard to the shading agent, a black material which can absorb light over the entire visible radiation range, such as carbon black, is especially useful, but a black mixture prepared from single color dyestuffs such as yellow, magenta, cyan and the like may also be used. Though any water soluble or water insoluble polymer may be used as a binder in the dispersion or addition of the shading agent, it is desirable that the black layer be further superposed with another layer in which a white pigment is dispersed, because it is preferable to allow for writing (as by a pencil) on the backing layer and it is not preferable from an aesthetic point of view to have a black layer as the outermost layer. For this purpose, the use of water soluble polymers is preferable to the use of water insoluble polymers from an industrial point of view, because simultaneous coating of two or more layers can be carried out easily by dispersing or adding a shading agent and a white pigment in or to a water soluble polymer, especially gelatin.
However, the aforementioned image receiving element for use in silver salt diffusion transfer processes, in which a cellulose ester or regenerated cellulose is coated on the front side (developer-contacting side) of the image receiving element and a gelatin-containing layer is coated on the back side, has a disadvantage in that prints made from such an element are apt to curl and lose their flatness, because the materials comprising the front side layers and back side layers have greatly different degrees of expansion and contraction due to humidity and temperature, especially humidity.
In an image receiving element in which a cellulose ester is used, the hydrolyzing degree of the ester changes depending on environmental conditions and over time due to the presence of the alkali in the developing solution spread. In addition, an image receiving element containing acetyl cellulose is apt to shrink because acetyl cellulose releases acetic acid by alkali hydrolysis, which in turn causes a decrease in the volume of the element.
Curling of photographic prints causes inconveniences in many ways. For example, curled prints make it difficult to observe the image. Curled prints are difficult to mount for preservation as records, and are also difficult to file in a pasted form on a mount because the curled print tends to bow the mount.
This invention contemplates overcoming the aforementioned problems involved in the prior art.